03 · The jet
Engines
Isp is how much Δv each kilogram buys. Thrust-to-weight is whether you leave the pad this afternoon. Chemical and ion pick opposite corners.
In English
Isp says how thrifty the propellant is. Thrust-to-weight says whether you can stand on the plume. A Merlin leaves the pad. An ion engine is a patient whisper for deep space.
- Try this
- Click Methalox, then Ion. Note Δv at the same mass ratio — and whether T/W is a liftoff number.
- Keep this
- First stage is T/W. Coasting between planets is Isp.
Lab · Isp vs thrust
Exhaust velocity (Isp) is how much Δv each kilogram of propellant buys. Thrust-to-weight is whether you leave the pad this afternoon. You do not get both.
Raptor, BE-4
- Isp
- 350 s
- Δv at ratio 6
- 6.15 km/s
- Vehicle T/W
- 140
Chemical · minutes to LEO. Gravity losses eat the slow ones.
Chamber pressure, mixture ratio, and nozzle expansion set Isp. Sea-level nozzles overexpand in vacuum; vacuum nozzles separate at the pad. The lab hides that on purpose.
Electric propulsion is for changing orbits after you are already up. It is not how you leave the pad. Photon thrusters are not in this volume.